Yep, that's definitely the assumption. However, I think it's also worth noting that zero-days, once disclosed, do typically receive advisories. Those advisories then (at least in Dependabot) bypass any cooldown controls, since the thinking is that a known vulnerability is more important to remediate than the open-ended risk of a compromised update.

> I'm sure the majority of bugs and vulnerabilities were never supply chain attacks: they were just ordinary bugs introduced unintentionally in the normal course of software development.

Yes, absolutely! The overwhelming majority of vulnerabilities stem from normal accidental bug introduction -- what makes these kinds of dependency compromises uniquely interesting is how immediately dangerous they are versus, say, a DoS somewhere in my network stack (where I'm not even sure it affects me).